The invention relates to a circulating grinding plant for the grinding of raw material, with a high-pressure roller mill or roller press with product bed comminution of the feed material and with a sifter device, comprising a static cascade sifter, to which at least one additional sifter is subsequently added on the sifting air current side, whose oversize material discharge is connected to the material feed of the roller press.
Pressure comminution of granular grinding product in the gap between two press rollers is known in the manner that the individual particles of the grinding product are mutually crushed upon application of a high pressure in a material bed compressed between the two roller surfaces, so that in this case one speaks of the so-called product bed comminution. The compression stress results partly in a particle destruction of the grinding product, and partly in the generation of cracks in the particle interior and manifests itself visibly in the formation of agglomerates (so-called scabbing), which can be deagglomerated or dissolved with comparably low expenditure of energy, so that this type of comminution is characterized overall by a comparably low specific power consumption.
In practice such a product bed roller mill or roller press is interconnected with a sifter device to a circulating grinding plant. For example a circulating grinding plant is known both from DE-A-42 23 762 FIG. 2 as well as from EP-B-0 650 763 with a roller press, static cascade sifter and with a subsequent sifter. In the process the static cascade sifter employed as a preliminary sifter and the subsequent sifter subsequently added on the sifting air current side, which can be a dynamic or a static sifter, are each combined into a compact constructional unit. The raw material is fed into the grinding circuit, in particular by means of feeding to the roller press or by means of feeding to the preliminary sifter, and the finished fine processed material is discharged out of the grinding circuit, to be precise by means of the sifter gas stream exiting the subsequent sifter. The cascade sifter acts as a preliminary sifting chamber for the grinding product directed into the circuit as well as acting simultaneously as a deagglomerator for dissolution of the pressure scabs still present in the recycle material so that a dynamic rod basket sifter subsequently added as a subsequent sifter with its rotating turbo elements is no longer loaded with coarse grain components. Both the coarse grain fraction from the static cascade sifter as well as the coarse grain fraction separated from the grinding product by the subsequent sifter can be re-circulated to the roller press for the purpose of repeated product bed stress and further grinding. Depending on the recirculation quantities, the rate of circulation in such a grinding circuit can be high. High grinding circuit rates, i.e., great quantities of circulating scabbing material or great quantities of a mixture of scabbing and raw material cause a considerable expenditure for the mechanical transport of this material to the sifter device, whereby the transport device as a rule consists of a bucket conveyor, which causes not inconsiderable investment and operating expenses.